1. Field of the Invention
The invention relates to an axial force measurement method for measuring an axial force of a bolt fastened to an object to be fastened.
2. Description of Related Art
The axial force of a bolt fastened to an object to be fastened is measured to, for example, verify the fastened state of the bolt. There is a correlation between the axial force and the displacement amount of the bolt head (that is, the difference in the recession amount of the head between the states before and after fastening). Therefore, the axial force is typically measured by acquiring the correlation between the axial force and the displacement amount of the head in advance, measuring the displacement amount of the head when actually measuring the axial force, and then measuring the axial force on the basis of the correlation.
In “A Measurement Method of Bolt Loads Based on Deformation of a Top Surface of a Bolt Head”, Transactions of the Japan Society of Mechanical Engineers. C 73(733), 2612-2618, the surface shape of the head is measured and the displacement amount of the head is measured on the basis of the difference between the displacement amount of the center (of gravity) of the head and the displacement amount of both ends. In “A Measurement Method of Bolt Loads Based on Deformation of a Top Surface of a Bolt Head”, Transactions of the Japan Society of Mechanical Engineers. C 73(733), 2612-2618, the measurement results on the surface shape of the head are approximated by a six-order polynomial, and the displacement amount of the head is calculated from the approximation result in order to suppress the error in calculation of the displacement amount of the head caused by irregularity resulting from surface roughness.
However, in this case, it is not unlikely that the measurement results on the displacement amount at both ends cannot be accurately approximated, that is, the end portions obtained by approximation cannot be accurate. Therefore, with the technique disclosed in “A Measurement Method of Bolt Loads Based on Deformation of a Top Surface of a Bolt Head”, Transactions of the Japan Society of Mechanical Engineers. C 73(733), 2612-2618, the displacement amount of the head cannot be measured accurately. As a result, it is not unlikely that the axial force cannot be measured accurately.
Further, with the technique disclosed in “A Measurement Method of Bolt Loads Based on Deformation of a Top Surface of a Bolt Head”, Transactions of the Japan Society of Mechanical Engineers. C 73(733), 2612-2618, the displacement amount of the head is measured from the displacement amount in three locations, namely, in the center and at both ends of the head. Therefore, with the technique disclosed in “A Measurement Method of Bolt Loads Based on Deformation of a Top Surface of a Bolt Head”, Transactions of the Japan Society of Mechanical Engineers. C 73(733), 2612-2618, where the measurement locations at both ends of the head are changed, the measurement results for the displacement amount of the head can change under the effect of the surface roughness of the head or irregularity formed at the head, even when the axial force is the same. In other words, with the technique disclosed in “A Measurement Method of Bolt Loads Based on Deformation of a Top Surface of a Bolt Head”, Transactions of the Japan Society of Mechanical Engineers. C 73(733), 2612-2618, it is not unlikely that the axial force cannot be measured stably.